The proportion of lymphocytes bearing the NKH-1 membrane marker (a natural killer-cell marker) was determined by flow cytometric analysis of the peripheral blood of 40 healthy individuals, 45 patients with multiple sclerosis and 19 with other neurological diseases. Natural killer (NK) cell activity was determined in parallel. It was shown that NKH-1-positive cells were more abundant in the lymphocytic than the mononuclear cell window on the flow-cytometric granularity versus cell size two-parameter display. NK-cell concentration did not correlate with NK-activity in any of the three groups of individuals studied. However, when the data for the three groups were compounded (i.e. when the number of samples analysed exceeded 100) significant correlations were disclosed. No correlation was found between age and NKH-1-positive cell number or activity in any of the three groups of subjects. High variability of the number and activity of natural killer cells in different individuals, and imprecision of phenotyping of the entire pool of NK-cells by means of a single marker, appeared to be responsible for poor correlations of NK-cell number and activity as observed in the study.